1. Field of the Invention
This invention relates to an apparatus and method for performing minimally invasive ocular laser treatments, and more particularly to an ophthalmic laser device configured for activating localized photothermal and/or photochemical processes while affected electrophysiological functions are maintained without exceeding pre-settable thresholds of change.
2. Description of the Related Art
There are several pathologies of the eye that cause some form of visual impairment up to and including blindness. A number of pathologies are currently treated with lasers such as glaucoma and retinal disorders. Retinal disorders treatable with laser include diabetic retinopathy, macular edema, central serous retinopathy and age-related macular degeneration (AMD).
Diabetic retinopathy represents the major cause of severe vision loss (SVL) for people up to 65 years of age, while AMD represents the major cause of SVL in people over 65 years of age. More than 32,000 Americans are blinded from diabetic retinopathy alone, with an estimated 300,000 diabetics at risk of becoming blind. The incidence of AMD in the USA is currently estimated at 2 million new cases per year, of which 1.8 million are with the xe2x80x9cdryxe2x80x9d form and 200,000 are with the xe2x80x9cwetxe2x80x9d form, also defined as choroidal neovascularization (CNV). CNV causes subretinal hemorrhage, exudates and fibrosis, any of which can lead to SVL and legal blindness.
A widely used form of laser treatment for retinal disorders is called laser photocoagulation (P.C.). Laser P.C. has become the standard treatment for a number of retinal disorders such as diabetic retinopathy, macular edema, central serous retinopathy, retinal vein occlusion and CNV. Laser P.C. is a photo-thermal process, in which heat is produced by the absorption of laser energy by targeted tissues, for the purpose of inducing a thermal xe2x80x9ctherapeutic damagexe2x80x9d, which causes biological reactions and, ultimately, beneficial effects. Conventional retinal P.C. relies on some visible xe2x80x9cblanchingxe2x80x9d of the retina as the treatment endpoint and can be defined as Ophthalmoscopically Visible Endpoint Photocoagulation (or OVEP) treatment. Since the retina is substantially transparent to most wavelengths used in laser P.C., its xe2x80x9cblanchingxe2x80x9d is not caused directly by the laser. Visible xe2x80x9cblanchingxe2x80x9d is the sign that the normal transparency of the retina has been thermally damaged by the conduction of heat generated underneath the retina in laser absorbing chromophores (i.e. melanin) contained in the retinal pigment epithelium (RPE) and in choroidal melanocytes.
The thermal gradient or elevation can be controlled by the laser (i) irradiance (power density), (ii) exposure time and/or (iii) wavelength. High thermal elevations are normally created with current OVEP clinical protocols that aim to produce visible endpoints ranging from intense retinal whitening (full thickness retinal burn) to barely visible retinal changes. Using the endpoint of visible retinal blanching is a practical way to assess the laser treatment, but it also constitutes disadvantageous and unnecessary retinal damage, which in turn results in a number of undesirable adverse complications including some vision loss, decreased contrast sensitivity and reduced visual fields in a substantial number of patients.
The damage of intense laser burns may also trigger neovascularization, a serious and highly undesirable event leading to further loss of vision. Due to the drawback of iatrogenic visual impairment due to thermal damage to the neurosensory retina, conventional OVEP laser treatment is presently considered and administered only late in the course of the disease, when it has become xe2x80x9cclinically significantxe2x80x9d and the benefit-to-risk ratio justifies the risk of associated negative effects. Recent clinical studies have suggested that patients with certain types of disorders could benefit from earlier treatment.
Various lasers procedures, referred to as minimum intensity photocoagulation (MIP), are now pursuing the beneficial therapeutic effects with invisible very light treatments with the goals to minimize iatrogenic retinal damage and to maximize the preservation of retinal tissue and visual functions. Less damaging MIP could be administered earlier in the course of the disease to patients with less compromised vision and with overall better results. For example, MIP is now experimentally administered to patients diagnosed with xe2x80x9cdryxe2x80x9d AMD presenting with high-risk drusen, as a prophylactic treatment to prevent or delay SVL due to the progression toward the xe2x80x9cwetxe2x80x9d neovascular stage. Another such example is transpupillary thermotherapy (TTT) (Reichel et al., 99) for the treatment of subfoveal occult CNV, a condition previously left untreated until it progresses into the visually devastating classic CNV.
All these treatments avoid visible retinal laser burns and can be defined as Non Ophthalmoscopically Visible Endpoint Photocoagulation or NOVEP treatment, to differentiate from the conventional OVEP treatment. Unfortunately, the absence of a visible endpoint during the laser treatment makes it difficult to select the proper irradiation dosage for each individual patient and leaves the physician with no tangible sign of having achieved the proper threshold for the minimum therapeutic damage (MTD). To make these treatments more popular and consistent, there is a need for a device and a method that allows intra-operative monitoring of sub-clinical changes during the laser treatment, able to provide the doctor with information about the treatment""s effects and/or to control and terminate the laser emission at a given pre-settable threshold of functional change. This would significantly decrease the difficulty associated with NOVEP procedures and would favor the acceptance by the ophthalmic community.
Recording of intra-operative electro-retinal functional changes can be performed using ElectroRetinoGram, Focal ElectroRetinoGram, or Multi-Focal ElectroRetinoGram. All of these are retinal evoked potential signals and will be referred to, collectively, as FERG. FERG can be spontaneous or elicited by flickering light stimulation and non-invasively detected and recorded through skin electrodes. The FERG has proven to be a sensitive indicator of macular cone system dysfunction in different retinal degenerative diseases (Seiple et al., 1986; Falsini et al., 1996), including age-related macular degeneration (Sandberg et al., 1993; Falsini et al., 1998). The FERG signals generated by flicker stimulation can be recorded and evaluated in terms of reliability and statistical robustness by steady-state, frequency-domain analysis techniques (Porciatti et al., 1989; Falsini et al., 2000). In addition, real-time retrieval and analysis of the responses to a set of stimulus parameters (sweep techniques) can be employed in a clinical setting to evaluate macular dysfunction (Seiple et al., 1993; Falsini et al., 2000).
To gather information from FERG signals a discrete Fourier analysis (Fadda et al., 1989) is performed on the average signal of multiple FERG responses to isolate the FERG fundamental harmonic. Amplitude (in microvolts) and phase (in degrees) can then be determined. Standard errors of the amplitude and phase estimates, derived from the block averages, are then calculated to determine response reliability. Averaging and Fourier analysis is also performed on signals sampled asynchronously from the temporal frequency of the stimulus, to derive an estimate of the background noise at the fundamental component. These FERG signals could be used to determine changes to the overlying neurosensory retina by monitoring the signal responses before treatment at a baseline level and then during treatment. Changes in electrophysiological signals would indicate changes to the patient""s retina caused by laser treatment.
The acceptance and adoption of NOVEP treatments by the ophthalmic community could be facilitated and accelerated if new user-friendly laser devices were available to allow the safe and consistent administration of NOVEP treatments. MIP treatment protocols would potentially be administered early in the course of the disease with better outcomes and would become the new standard-of-care. It is proposed by this patent that a method and apparatus utilizing intra-operative monitoring of electro-physiological changes would allow a laser surgeon to more easily perform NOVEP treatments. Accordingly, there is a need for FERG recorded through the use of electrodes in order to provide the real time monitoring of intra-operative electrophysiological functional changes from baseline.
Accordingly, an object of the present invention is to provide a system, and its method of use, that uses FERG recorded through the use of electrodes in order to provide the real time monitoring of intra-operative electrophysiological functional changes from baseline.
Another object of the present invention is to provide a system, and its method of use, for performing clinically effective NOVEP laser treatment with the minimum possible thermal damage to the overlying neurosensory retina by monitoring FERG signals before, during, and after treatment.
These and other objects of the present invention are achieved in a method of monitoring and controlling the sub-threshold laser treatment of a patient""s retina. Sensors are located on a patient to measure focal electroretinograms (FERG). A stimulating beam is delivered onto the patient""s retina. A pre-treatment FERG signal is collected. Treatment FERG signals are collected while treating the retina with a sub-threshold laser treatment. A difference is determined between the baseline and treatment FERG signals. The difference is used to control the termination of the treatment.
In another embodiment of the present invention, a method of monitoring and controlling the sub-threshold laser treatment of a retina locates electrode sensors on a patient to measure FERGs. A stimulating beam is delivered onto the patient""s retina. A pre-treatment FERG signal is collected. Treatment FERG signals are collected while treating the retina with a sub-threshold laser treatment. A difference is between the pre-treatment and treatment FERG signals. The difference is used by a physician to control a termination of the treatment.
In another embodiment of the present invention, a system for monitoring and controlling the sub-threshold laser treatment of a patient""s retina includes at least two sensors to measure FERGs. A laser delivery system includes a laser treatment beam, an aiming beam, and a stimulating beam, each co-aligned onto the retina. Resources convert the FERG signal changes into parameters to control the laser for an optimum sub-threshold treatment.